yes it is Jap Knotweed. You may find the local council will help solve the problem. if not then a continued attack on it with glyphosphate type weedkiller.
do not compost it or add it to greenwaste. If you can burn it.

Yes, I am afraid that this is Japanese Knotweed.

Be very careful about this. Japanese knotweed can make it very difficult to sell your home if a surveyor spots it( assuming you are not renting)

I would treat it repeatedly with round up and if possible digging out until it is gone .

If it is in your neighbour's gardens then you'll need to try to arrange a concerted attack on it

there is a round up stump remover that can kill jap knotweed as well, i have used it on bramble roots and it worked fine.

Thanks for the confirmation. Been quoted about £2,500-£3,000 to get rid of over the next 4 years with glyphosphate.
Think it may be cheaper to treat it myself and luckily it is at the back of the garden.
There is a massive line of Knotweed in my neighbors garden running along the back fence so will have to check what they plan to do

The quote of £2500-3000 is ridiculous!

I buy concentrated roundup for about £45 for 5 litres and that keeps me going for at least 4 years and I'm a professional gardener doing 35 gardens a week !

If there is a 'massive line' of knotweed in your neighbours garden then you will have a lifetime job keeping it out of yours. i would suggest contacting the local authority about this and see if they are interested in taking any action.

In the meantime if there isn't very much (yet!)Try cutting each stem down and pouring neat weedkiller down into the hollow stem - this can work quite well.